Colorectal cancer is the third leading cause of cancer-related deaths in the United States with 130,000 new cases diagnosed per year and approximately 57,000 deaths expected in 2004. A better understanding of the signaling pathways contributing to colorectal cancer progression will provide targets for novel therapeutic agents to treat this disease. Approximately 85% of colorectal cancers have mutations of the APC (adenomatous polyposis coil) tumor suppressor gene, and about 13% of colorectal cancers have mutations of the B-catenin oncogene. Mutations in APC genes result in B-catenin accumulation that activates other oncogenes. B-catenin stability is controlled by N-terminal serine/threonine phosphorylation. The phosphorylated B-catenin is degraded by the ubiquitin/proteasome pathway. B-catenin mutations that escape phosphorylation or ubiquitination result in abnormal B-catenin accumulation that lead to cancer. Conversely, the molecular mechanisms of APC mediated B-catenin degradation are not clear. Most APC mutations in colon cancers are C-terminal truncations; APC truncations missing different functional domains result in varying levels of tumor differentiation. The central hypothesis of this proposal is that APC regulates B-catenin degradation by regulating both B-catenin phosphorylation and ubiquitination. To examine our hypothesis, we have designed experiments with the following Specific Aims: 1) to delineate mechanisms of APC mediated regulation of B-catenin phosphorylation, and 2) to assess the mechanisms contributing to APC regulation of B-catenin ubiquitination. B-catenin phosphorylation and ubiquitination are regulated by distinct domains of APC. Understanding these regulations at a molecular level is crucial for rational drug design for colon cancer therapeutics.